The aluminium that retail chassis are made from is a joke. Get some
1.6mm 5052-H14 from a local aluminium shop, a $30 brake and away you go.
You can make 5-7 for the cost of one retail ;-)

A factor of 5:1 in price?

My local sources charge about $5 per sq ft for 0.040" 5052-H14
aluminum.

A 17x10x3 chassis needs about 3 square feet.

That's $15 for materials, not counting wastage because the local
source only sells it in 4x8 feet sheets :-). I compare that to $25 for
a Hammond chassis of the same size already made and spot welded, and I
don't see as big (5:1 or 7:1) of a disparity in prices as you do.

Now, I can get little (square foot or two) aluminum cutoffs at some
scrap piles for a lot cheaper, and those work great for tiny little
subchassis or miniboxes, even better to do it myself there because
dimensions are often critical.

Maybe I'm buying really super expensive aluminum and you know where to
get it for $1 or so a square foot. If so, tell me where!
==========================
Over the years I bought some large size sheets double sided PCB material
at flea markets.
Provided there are no heavy transformers ,both chassis and enclosure can
be made of the material.
Under chassis screened compartments can be easily included.


Frank GM0CSZ / KN6WH